"Banding" is the term used to refer to the effect obtained in raster scanning systems when the spacing between each raster line in the image recorded thereby is not equal. In those scanning systems which employ multi-faceted scanners, a primary cause of banding is due to differences between the angles of each facet of the scanner relative to the axis of rotation of the scanner.
It has been demonstrated that scanners having angle tolerances of .+-.2 arc seconds between facet angles can be constructed relatively easily and inexpensively, but the construction of a scanner having facets positioned to a significantly greater accuracy can be achieved, if at all, only at great expense.
There are many applications, however, in which a reduction in banding, and, hence, more accurate facet alignment is necessary or, at least, would be very desirable. For example, it would be desirable to be able to provide a relatively inexpensive, easy to construct scanner in which the difference between facet angles is no more than about .+-.0.5 arc seconds.
The problem of banding has, of course, been recognized previously, and there are a variety of techniques suggested in the prior art to reduce or eliminate it. U.S. Pat. No. 3,657,792, for example, describes a system and procedure by which each facet in a multi-faceted scanning mirror can be individually adjusted. This technique, and, in fact, other correction schemes involving the direct adjustment of mirror elements are not very good because only minute changes in mirror position will cause relatively significant changes in the position of the output light beam making accurate adjustment very difficult and time consuming.
Other suggested correction techniques involve the use of active systems such as acousto-optic beam deflectors, and, while such an approach appears to be feasible, it is relatively complex and expensive due to the need for electronics and other costly components.
U.S. Pat. No. 4,129,355 describes another system using a pair of self-balancing multi-faceted scanning mirrors to apparently eliminate parallelism errors by subtractive cancellation, and in column 1 of this particular patent yet additional approaches are discussed.
In general, the various techniques suggested in the prior art are lacking in that they appear to be expensive, complex or of doubtful accuracy.